The Carrington event of August/September 1859 was the most significant solar proton event (SPE) of the last 450 years, about four times larger than the solar proton fluence of the largest event from the “spacecraft era” (August 1972). Recently, much attention has focused upon increasing our understanding of the Carrington event, in order to better quantify the impact of extreme space weather events. In this study the Sodankyla Ion and Neutral Chemistry (SIC) model is used to estimate the impact of the Carrington event to the neutral atmosphere and the ionosphere, and the disruption to HF communication. We adopt a reported intensity-time profile for the solar proton flux and examine the relative atmospheric response to different SPE-energy spectra, and in particular, the comparatively soft energy spectrum of the August 1972 or March 1991 SPE which is believed to provide the best representation of the Carrington event. Our calculations indicate that large changes in electron density and atmospheric constituents occur during the period of SPE forcing, depending upon the nature of the spectrum and also on the hemisphere considered. However, the most important SPE-driven atmospheric response is an unusually strong and long-lived O-x decrease in the upper stratosphere (O-x levels drop by similar to 40%) primarily caused by the very large fluxes of >30 MeV protons. This depletion is an indication of the extreme changes possible for the largest SPE. We find that there are comparatively small long-term differences in the atmospheric and ionospheric response between the 3 suggested SPE spectra.